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From the design perspective, how can the torque of a squirrel-cage induction motor be increased?
1. By increasing the supply voltage
2. By increasing the rotor resistance
3. By increasing the rotor reactance
4. By increasing rotor EMF

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Correct Answer - Option 2 : By increasing the rotor resistance


The torque equation of a three-phase induction motor is given by,

\(T = \frac{{180}}{{2\pi {N_s}}}\left( {\frac{{sE_2^2{R_2}}}{{\left( {R_2^2 + {s^2}X_2^2} \right)}}} \right)\)

Where Ns is the synchronous speed

E2 = rotor emf

R2 = rotor resistance

X2 = rotor reactance

s is the slip

By the above expression, we can say that the torque of an induction motor depends on rotor resistance, supply voltage, and slip.

From the design perspective, the torque can be increased by increasing the rotor resistance.

Note: While designing, if we keep the more rotor resistance, the torque will increase. But, we can't add extra resistance in the rotor circuit in the case of a squirrel cage motor.

The method of adding extra rotor resistance is applicable to W.R.I.M. due to wound rotor is short-circuited with slip rings, where external resistance is added through the brush whereas, in squirrel cage induction motor, the rotor is made of non-insulated copper conductors in rod or bar formats & short-circuited with end rings.

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